The Earth is not the steady round rock we think it is, and it's not turning the graceful circles we imagine. Every little-bit-more-than-a-year, we make a sort of wobble in space called a nutation. Why?

Imagine a top spinning in front of you. At first, when it spins quickly, it's steady on its tapered base. If you play with it a little, nudging it one way or another, the handle on its top will dip and go around in a slow circle. The Earth makes the same kind of motion. The most famous version of that motion is called precession. It makes a circle motion in space, completing a cycle every 26,000 years. That's a large difference, changing the apparent position of stars. (If you're 72 years old, the earth has completed one degree of precession during your lifetime. Congrats!)

If you don't have 26,000 years to watch the Earth make a big loop, there's a smaller and shorter process. Along with a relatively big circle, the Earth makes a little wobble – a nutation. The Earth bulges around its center, and the moon, the sun, and the planets nudge at that little bulge. The total amount of nudging would amount to a tiny wobble of the Earth every 305 days. In fact, it takes about 435 days for the Earth to complete a nutation, because the Earth isn't a solid object. Its insides and outsides are sloshing with liquid, and cause the nutation to lag just a bit. So, if the year is going a little too fast for you, just celebrate the nutation of the Earth instead of the orbit of the Earth around the sun. It's just as legitimate.


Image: NASA.

[Via A Normal Mode Study of Wobble and Nutation, University of Alberta]